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Mark Russinovich's blog is very technical and comes highly recommended. You'll learn a lot about how Windows actually works including lots of examples of troubleshooting difficult real world problems.

Mark explores (and solves) lots of problems that many would explain by saying things like 'Windows slows down if you've had it installed for a long time'. Those same people will then tell you that reinstalling / rebooting is the best solution.

Marcus J Ranum's site is sort of a blog and has valuable if infrequently updated information: http://www.ranum.com/

Uber Geek Brian K Jones has a blog at his amusingly named domain. It delves into mostly *nix issues. He is responsible for introducing me to and convincing me of the concept that SysAdmins should be proficient in at least one programming language: http://www.protocolostomy.com/

Bruce Schneier's blog and Crypto Gram newsletter is a must for anyone interested in security. It doesn't just focus on computer security, but a much more comprehensive thought process. Watching Schneier and Ranum face off is an enlightening experience. http://www.schneier.com/blog/

Basically, peruse the many blogs in the Planet SysAdmin blog roll. In fact, I see at least one person from that blog roll on this site. ::waves at Matt Simmons::

I despise MS documentation as it's generally written in some oddball bureaucratese language with lots of circular references. I find the folks writing the blogs to be a breath of fresh air as they actually write in English.